You can do both. One way that works well is to split up the season. Spend the first portion bike racing with a very light focus on swim and run. Later in the season move on to Xterra focused training using bike fitness built up to your advantage. It really depends on the timing of the events you want to focus on.
If you have your swim skills established it is easy to insert a couple of swims per week into any training plan as recovery sessions. Runs are harder to fit in. That is where you have to make decisions about which rides to take out of the plan and replace with runs or brick sessions.
…and who said anything about dropping intensity when training for stage races or 100 milers? You need to have intensity in your schedule in order to build your fitness up to the level needed to make the time cut offs in some events let alone be among the podium group. All endurance plans have intensity in them.
I do have a 12 week Xterra training plan available.